diff --git a/countdown_watchface/src/pkjs/index.js b/countdown_watchface/src/pkjs/index.js
index 4b8d2d2..d251449 100644
--- a/countdown_watchface/src/pkjs/index.js
+++ b/countdown_watchface/src/pkjs/index.js
@@ -77,10 +77,17 @@ Pebble.addEventListener('webviewclosed', function(e) {
   if (!e.response || e.response === 'cancel') return;
   try {
     var cfg = JSON.parse(decodeURIComponent(e.response));
-    localStorage.setItem('chunk', cfg.chunk);
-    localStorage.setItem('start', cfg.start);
-    localStorage.setItem('end',   cfg.end);
-    sendConfig(cfg);
+    var chunk = parseInt(cfg.chunk, 10);
+    var start = parseInt(cfg.start, 10);
+    var end   = parseInt(cfg.end,   10);
+    if (chunk <= 0 || chunk > 60) return;
+    if (start < 0 || start > 22)  return;
+    if (end < 1   || end > 23)    return;
+    if (start >= end)             return;
+    localStorage.setItem('chunk', chunk);
+    localStorage.setItem('start', start);
+    localStorage.setItem('end',   end);
+    sendConfig({ chunk: chunk, start: start, end: end });
   } catch (err) {
     console.log('Failed to parse config: ' + err);
   }